A good measure is its internal resistance. If the internal resistance was 1 ohm, the output voltage would fall by 1 volt for every amp of current taken. So - how low is the internal resistance? is a good question to find out how strong the source is.
load test
load test
voltmeter
voltmeter
The battery is the voltage or power source, the wires form the conductor.
Consider a battery for a moment. A battery can be a source or sink type of power source. A light bulb would be considered a source current. When a battery or other item is being charged is it considered a sinking source.
The battery is the voltage or power source, the wires form the conductor.
Battery
Car battery.
battery
Your phone has a battery behind back cover as a power source.
Disconnect the negative battery cable.